Cieslewicz M J, Kasper D L, Wang Y, Wessels M R
Channing Laboratory, Harvard Medical School, Boston, Massachusetts 02115, USA.
J Biol Chem. 2001 Jan 5;276(1):139-46. doi: 10.1074/jbc.M005702200.
Several species of streptococci produce extracellular polysaccharides in the form of secreted exopolysaccharides or cell-associated capsules. Although the biological properties and repeating unit structures of these polysaccharides are diverse, sequence analysis of the genes required for their production has revealed a surprising degree of conservation among five genes found in the capsule gene cluster of each of several polysaccharide-producing streptococci. To determine the function of these conserved genes, we characterized a series of isogenic mutants derived from a wild-type strain of type Ia group B Streptococcus by selectively inactivating each gene. Inactivation of cpsIaE resulted in an acapsular phenotype, consistent with previous work that identified the cpsIaE product as the glycosyltransferase that initiates synthesis of the polysaccharide repeating unit. Mutants in cpsIaA, cpsIaB, cpsIaC, or cpsIaD produced type Ia capsular polysaccharide, but in reduced amounts compared with the wild type. Analysis of the mutant polysaccharides and of capsule gene transcription in the mutant strains provided evidence that cpsIaA encodes a transcriptional activator that regulates expression of the capsule gene operon. Mutants in cpsIaC or cpsIaD produced polysaccharide of reduced molecular size but with an identical repeating unit structure as the wild-type strain. We conclude that CpsA to -D are not required for polysaccharide repeating unit biosynthesis but rather that they direct the coordinated polymerization and export of high molecular weight polysaccharide.
几种链球菌以分泌型胞外多糖或细胞相关荚膜的形式产生胞外多糖。尽管这些多糖的生物学特性和重复单元结构各不相同，但对其产生所需基因的序列分析显示，在几种产多糖链球菌各自的荚膜基因簇中发现的五个基因之间存在惊人程度的保守性。为了确定这些保守基因的功能，我们通过选择性地使每个基因失活，对源自Ia型B组链球菌野生型菌株的一系列同基因突变体进行了表征。cpsIaE失活导致无荚膜表型，这与之前将cpsIaE产物鉴定为启动多糖重复单元合成的糖基转移酶的研究结果一致。cpsIaA、cpsIaB、cpsIaC或cpsIaD的突变体产生Ia型荚膜多糖，但与野生型相比产量降低。对突变体多糖和突变菌株中荚膜基因转录的分析提供了证据，表明cpsIaA编码一种转录激活因子，可调节荚膜基因操纵子的表达。cpsIaC或cpsIaD的突变体产生的多糖分子量降低，但重复单元结构与野生型菌株相同。我们得出结论，CpsA至 -D不是多糖重复单元生物合成所必需的，而是它们指导高分子量多糖的协调聚合和输出。
